Pointer to the starting address of an array of local values.
On return, the data array on the root will be overwritten
with the result of the reduce operation over the group.
For the other (non-root) members of the group the values
of the data array upon return from the reduce operation
are not defined; the values may be different than
those originally passed to pvm_reduce.

count

Integer specifying the number of elements of
datatype
in the data array.
The value of count should agree between all members of the group.

datatype

Integer specifying the type of the entries in the data array.
(See below for defined types.)

msgtag

Integer message tag supplied by the user.
msgtag should be >= 0. It allows the user's program to
distinguish between different kinds of messages.

group

Character string group name of an existing group.

rootginst

Integer instance number of group member who gets the result.

info

Integer status code returned by the routine.
Values less than zero indicate an error.

DESCRIPTION

pvm_reduce()
performs global operations such as max, min, sum, or a
user provided operation on the data provided
by the members of a group.
All group members call pvm_reduce with the same size local
data
array which may contain one or more entries.
The
root
task is identified by its instance number in the group.

The inner workings of the pvm_reduce call are implementation dependent;
however, when the pvm_reduce call completes, the root's data
array will be equal to the specified operation applied element-wise to
the data arrays of all the group members.

A broadcast by the root can be used if the other members of the group
need the resultant value(s).

PVM supplies the following predefined functions
that can be specified in
func.

PvmMin
PvmMax
PvmSum
PvmProduct

PvmMax and PvmMin are implemented for all the datatypes listed below.
For complex values the minimum [maximum] is that
complex pair with the minimum [maximum] modulus.
PvmSum and PvmProduct are implemented for all the datatypes listed
below with the exception of PVM_BYTE and BYTE1.

A user defined function may be used in
func.
The argument func is a function with four arguments. It is the base
function used for the reduction operation. Both x and y are
arrays of type specified by datatype with num entries.
The arguments datatype and info are as specified above.
The arguments x and num correspond to data and count above.
The argument y contains received values.

Caveat: pvm_reduce() does not block, a call to pvm_barrier may be necessary.
For example,
an error may occur if a task calls pvm_reduce and then leaves the group
before the root has completed its call to pvm_reduce.
Similarly, an error may occur if a task joins the group after the root
has issued its call to pvm_reduce.
Synchronization of the tasks (such as a call to pvm_barrier) was not
included within the pvm_reduce implementation since this overhead
is unnecessary in many user codes (which may already synchronize
the tasks for other purposes).

The current algorithm is very simple and robust. A future implementation
may make more efficient use of the architecture to allow greater
parallelism.

ILLUSTRATION

The following example illustrates a call to pvm_reduce.
Suppose you have three group members (instance numbers 0, 1, 2)
with an array called Idata with 5 values as specified: